This application is a 371 of PCT/EP99/08536 filed Nov. 8, 1999.
The present invention relates to the production of tetrahydrofuran and gamma butyrolactone by hydrogenation of crude or refined maleic anhydride recovered from the off gases from the catalytic vapour phase oxidation of n-butane.
Due to the reduced production costs from n-butane, maleic anhydride (MAN) is a very attractive starting material for the production of derivatives like tetrahydrofuran (THF), gamma butyrolactone (GBL) and butanediol (BDO) by catalytic hydrogenation.
Several procedures have been disclosed for making GBL, THF and BDO via hydrogenation of MAN and/or carboxylic acids, expecially maleic acid. The following patents can be cited:
Several different catalysts have been proposed for the conversion of MAN into GBL,THF and/or BDO. As taught by these patents, a performant catalytic system comprises an element of the VII B group of the Mendeleev Periodic Table, preferably Rhenium or a compound thereof, associated with at least one noble metal of group VIII, preferably Palladium or a compound thereof, on a carbon or inert oxide support.
A catalyst of this kind has been first described by GB Pat. 1,551,741 and by DE Pat. 2,519,817.
As described in the technical and patent literature, MAN is fed to the hydrogenation preferably dissolved in a solvent.
U.S. Pat. No. 4,810,807 describes a hydrogenation process wherein MAN is dissolved in n-butanol.
DE Pat. No. 2,519,817 describes a process where MAN is dissolved in dioxane.
U.S. Pat. No. 4,782,167 describes a process wherein once dissolved in water, MAN is fed to hydrogenation as a maleic acid solution.
The use of organic solvents for dissolving MAN involves complex distillation steps in order to allow recovery, purification and recycling of the solvent.
On the other hand, the use of a maleic acid solution presents the advantage of simplifying the sequence of operations in the plant where butane is used as feedstock for the production of MAN.
The integration of the maleic anhydride process with the hydrogenation process results in a simplified maleic acid recovery (as opposed to MAN) and avoids MAN purification and refining.
Feeding maleic acid has, however, a negative impact on process economics since 10 to 15 mols of water are fed to the hydrogenation reactor, for each mol of MAN.
Compared with anhydrous MAN hydrogenation, where only about one mol of water forms for each mol of MAN converted to GBL or BDO, plus one additional mol per mol of THF formed, processing the reactor effluent in the presence of a large excess of water complicates THF recovery, GBL and/or BDO purification, and water separation.
Furthermore, maleic acid hydrogenation at relatively high temperature and pressure results in a corrosive environment requiring expensive construction material.
Therefore, the most important aim of the present invention is that of providing an optimized process for the production of GBL and THF, based on an efficient integration between the maleic anhydride process and the hydrogenation process, wherein the problems and troubles of using an aqueous maleic acid solution are solved to a great extent in an industrially convenient manner.
According to the present invention, the above aim has been accomplished by proposing a process for the production of MAN by n-butane oxidation in the vapour phase, wherein the oxidizing medium is air or, preferably, pure oxygen (or enriched air) mixed with reaction gases, and wherein MAN is recovered from the reaction gases by using an organic solvent as absorption medium, that is a solvent which is a MAN hydrogenation product at the same time, namely GBL.
The use as a solvent of a product from the process object of the present invention avoids all the drawbacks and costs associated with the recovery and purification of an additional chemical solvent which is extraneous to the process, and further avoids the high costs arising from the use of water as solvent which is described by other patents.
A further important feature of the process object of the present invention is that MAN, and not maleic acid, is essentially the major absorption product, despite the large amount of excess water which is present in the butane oxidation effluent.
Such a thing avoids the difficulties related to corrosion and also avoids the need of any expensive construction material, as it is the case when maleic acid is subjected to hydrogenation in the presence of water.
The process object of the present invention is further characterized by comprising the following operations:
a) Converting n-butane to MAN by catalytic vapour phase oxidation.
b) Recovering MAN from the effluent gases from the butane oxidation by selective absorption into GBL, forming a MAN-GBL mixture.
c) Removing water from the MAN-GBL mixture in a stripper, under the action of a gas and/or under vacuum, producing a MAN-GBL mixture with a minimum water and maleic acid content.
d) Recovering GEL from the exhaust gases that leave the maleic anhydride absorber, by water absorption;
e) Dehydrating the GBL recovered and recycling it to the maleic anhydride absorber;
f) Hydrogenating the dewatered MAN-GBL mixture over suitable catalyst(s) under conditions that favour THF and GBL formation.
g) Separating THF, GBL and by-products from the resulting hydrogenation mixture, by distillation.
h) Recycling a GBL rich stream to the MAN selective absorption.
The main advantages of the process object of the present invention can be summarized as follows:
a) It does not use an extraneous organic product as a solvent, such as dioxane, which would be difficult to recover and purify.
b) It avoids the use of a large excess of water as in the case of the processes wherein a maleic acid solution is used, and wherein high energy consumptions are required for water separation and THF and GBL recovery.
c) It avoids to feed corrosive maleic acid to the hydrogenation reactor, which would require expensive construction material, as for instance hastelloy.
d) A non refined fraction of the GEL product is used as solvent for the maleic anhydride absorption, allowing an optimum integration of the maleic anhydride process with the hydrogenation process.
e) THF and GBL are obtained from MAN in high yields.
Assuming that MAN is produced by a total recycle, high productivity process, wherein oxygen is used as the oxidizing medium, the process object of the present invention is able to produce GEL and THF with a consuption that approaches 1 Ton of butane per Ton of GBL equivalent, with reduced investment and optimized operating costs.
The sequence of the operations involved in the process object of the present invention is shown in FIG. 1.
The process of this invention is valid for any process wherein n-butane is converted to maleic anhydride by catalytic vapour phase oxidation.
The effluent gases from the maleic anhydride reactor (line 1) are conveyed to an absorber (2) where MAN is recovered by countercourrent washing with a GEL rich stream being recycled from the hydrogenation unit (line 3) and from a GBL stream recovered from the exhaust effluent gases (line 9).
In the lower section of the absorber (2), the resulting MAN-GBL mixture is contacted with a gas stream (line 4).
Under the gas and heat stripping action, most of the water countained in the MAN-GBL mixture is removed.
The outlet operating pressure at the absorber (2) ranges from 1.2 to 6.5 bar.
The operating temperature is controlled in such a way as to limit water condensation within the organic liquid effluent, being at least 10xc2x0 C. above water""s dewpoint.
The gases leaving the absorber (2) are conveyed (line 5) to the lower section of a scrubber column (6) where the GBL trapped by the exhaust gases is recovered by washing with water.
The aqueous GBL solution that leaves the water scrubber (6) is fed (line 7) to a dewatering column (8) which separates a GBL rich stream that flows back (line 9) to the absorber (2).
The overhead separation water from the dewatering column (8) is disposed of (line 10).
The excess water produced in the maleic anhydride converter (line 11) is removed in the upper section of the water scrubber (6).
The exhaust gases leaving the water scrubber (6) are recycled (line 12) to the the maleic anhydride plant reaction system.
The liquid effluent from the absorber(2), a MAN-GBL stream with a minimum water and maleic acid content, flows (line 13) to the hydrogenation unit.
Our concept of hydrogenation involves one or more, preferably two, catalytic stages.
In the hydrogenation unit, the DAN-GBL stream (line 13) is mixed with a preheated hydrogen stream (line 14) and both of them, after final preheating (15), flow (line 16) to the first hydrogenation stage (17).
The overall hydrogenation operating conditions and major performances are:
Total selectivity to GBL+THF: from 90% to 98% When more than one stage of catalyst is employed, the catalyst of the first stage will hydrogenate maleic anhydride into succinic anhydride and will also carry out part of the hydrogenation of succinic anhydride to gamma butyrolactone.
A preferred catalyst of the first stage of hydrogenation is a catalyst comprising a noble metal of group VIII of the Periodic Table, preferably palladium or a compound thereof, on a support, preferably carbon or an inert oxide.
For the purposes of this description such a catalyst will be identified as Pd catalyst. The quantity of catalytically active substance in the Pd catalyst is from 0.1% to 10% by weight of the total weight of the catalyst, preferably from 0.2% to 2% by weight. Another preferred catalyst of the first stage of hydrogenation will be a nickel based catalyst, which may be used with or without a carrier. For the purposes of this description such a catalyst will be identified as Ni catalyst. The quantity of catalytically active substance in the supported Ni catalyst is from 1% to 50% by weight of the total weight of the catalyst, preferably from 5% to 2% by weight.
A preferred catalyst which can be used to perform the hydrogenation comprises an element of group VII B of the Mendeleef Periodic Table, preferably rhenium or a compound thereof, associated with at least one noble metal of group VIII of such Periodic Table, preferably palladium or a compound thereof, on a support, preferably carbon or an inert oxide. For the purposes of this description, such a catalyst may be identified as Pd-Re catalyst.
The Pd-Re catalyst can be used in hydrogenation in either one of the following configurations:
a) Hydrogenation reaction consisting of one stage only
b) Hydrogenation reaction consisting of two or more stages
c) Hydrogenation reaction consisting of two or more stages where the first stage uses a Pd catalyst or a Ni catalyst.
The quantity of catalytically active substance on the Pd-Re catalyst is from 0.1% to 10% by weight of the total weight of catalyst, preferably from 1% to 5% by weight.
The weight ratio of element of group VII B, preferably rhenium, to the noble metal of group VIII, preferably palladium, is from 1:1 to 10:1, preferably from 2:1 to 5:1.
Other preferred catalysts used either to perform the hydrogenation in place of the Pd-Re catalyst in either one of the configurations referred to above, are catalysts of the palladium-nickel type or catalysts of the palladium-copper type. For the purposes of this description catalysts of palladium-nickel type will be identified as Pd-Ni catalysts, and catalysts of palladium-copper type will be identified as Pd-Cu catalysts.
The quantity of catalytically active substance in the Pd-Ni or in the Pd-Cu catalysts is from 1% to 5% by weight of the total weight of the catalyst preferably from 5% to 20% by weight.
The weight ratio of either nickel or copper to palladium in said catalysts is from 0.5:1 to 50:1, preferably from 5:1 to 20:1.
Using a Pd catalyst in the first stage of hydrogenation to saturate maleic anhydride into succinic anhydride and to convert part of succinic anhydride into gamma butyrolactone is advantageous due to the inferior cost of Pd catalyst compared to Pd-Re, Pd-Ni or Pd-Cu catalysts.
After each stage a stream of cold hydrogen is injected to control the temperature of reaction.
This is shown in FIG. 1 as a stream of hydrogen (line 18) entering the reactor after the first stage (17), cooling the mixture entering the second stage (19).
As an alternative the effluent from each stage may be cooled by indirect heat exchange.
The effluent of reaction (line 20) will contain, besides GBL and THF, hydrogen, water, light organics (such as propanol, butanol, propionic acid, butyric acid), heavy organics (succinic acid, butanediol).
The effluent is cooled first by exchanging heat (21) with hydrogen and then in the cooler (22).
The cooled effluent (line 23) enters the separator (24), from which a hydrogen rich stream flows overheads (line 25).
A small fraction of the hydrogen is purged (line 26) to avoid accumulation of inerts. The remaning fraction flows to the compressor (27).
Fresh hydrogen (line 29) joins the compressed hydrogen rich gas (line 28).
A portion of the compressed hydrogen flows (line 18) to the interstages of the reactor to control the temperature of reaction. Another fraction (line 30) is preheated (21) and mixed with the feed to the hydrogenation reactor.
The liquid phase leaving the separator (24) flows (line 31) to a fractionation unit (32) which separates THF product (line 33), water and light organics (line 34), GEL product (line 35), heavy ends (line 36). A crude GEL rich stream, containing some heavy organics, is recycled (line 3) to the absorber (2) of the maleic anhydride plant.
According to this concept neither heavies nor lights can accumulate in the crude GEL used as absorption medium.
The only purge necessary is in the standard purification of the products.
The embodiments of the present invention are evidenced by Example A hereinafter.